U.S. patent application number 09/985396 was filed with the patent office on 2002-05-23 for method and apparatus for wet-cleaning substrate.
Invention is credited to Inagaki, Yosuhito.
Application Number | 20020059943 09/985396 |
Document ID | / |
Family ID | 27531708 |
Filed Date | 2002-05-23 |
United States Patent
Application |
20020059943 |
Kind Code |
A1 |
Inagaki, Yosuhito |
May 23, 2002 |
Method and apparatus for wet-cleaning substrate
Abstract
In the art of wet-cleaning a substrate by etching with a
cleaning solution prepared by dissolving hydrofluoric acid as an
active component in water, using the process of measuring the
concentration of a predetermined component regularly and then
replenishing the cleaning solution with a component for correcting
the concentration at need on the basis of the result of measurement
on the concentration in case of cleaning the substrate with an
aqueous solution of ammonium fluoride as the cleaning solution
while controlling air in a cleaning draft at an exhaust rate within
a predetermined range.
Inventors: |
Inagaki, Yosuhito;
(Kanagawa, JP) |
Correspondence
Address: |
RADER, FISHMAN & GRAUER, P.L.L.C.
Suite 501
1233 20th Street, NW
Washington
DC
20036
US
|
Family ID: |
27531708 |
Appl. No.: |
09/985396 |
Filed: |
November 2, 2001 |
Current U.S.
Class: |
134/18 ;
134/100.1; 134/113; 134/36; 134/56R; 257/E21.528 |
Current CPC
Class: |
C11D 7/08 20130101; Y10S
134/902 20130101; C11D 11/0047 20130101; H01L 21/67086 20130101;
H01L 21/67057 20130101; B08B 3/08 20130101; H01L 22/26 20130101;
Y10T 436/12 20150115; C11D 7/10 20130101 |
Class at
Publication: |
134/18 ;
134/56.00R; 134/36; 134/100.1; 134/113 |
International
Class: |
B08B 003/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 2000 |
JP |
P2000-341094 |
Nov 8, 2000 |
JP |
P2000-341093 |
Nov 8, 2000 |
JP |
P2000-341092 |
Nov 29, 2000 |
JP |
P2000-363155 |
Dec 5, 2000 |
JP |
P2000-369632 |
Claims
What is claimed is:
1. A method for wet-cleaning a substrate through the process of
storing a cleaning solution prepared by dissolving active
components in water in a cleaning bath provided in a cleaning draft
and immersing substrates to be cleaned in the cleaning solution for
cleaning while allowing clean air to flow into the cleaning draft,
the method comprising the steps of: cleaning the substrates to be
cleaned while supplying clean air humidified to control a relative
humidity within a predetermined range toward a surface level of the
cleaning solution in the cleaning bath downwards vertically,
together with exhaust of air of the cleaning draft at an exhaust
rate in a predetermined range.
2. The method for wet-cleaning a substrate according to claim 1,
wherein the clean air to be supplied is limited in relative
humidity to the range of 40 to 50% and in temperature to a room
temperature.
3. The method for wet-cleaning a substrate according to claim 1,
wherein the cleaning solution prepared by dissolving at least one
of ammonium fluoride and hydrofluoric acid in water is used.
4. An apparatus for wet-cleaning a substrate through the process of
storing a cleaning solution prepared by dissolving active
components in water in a cleaning bath provided in a cleaning
draft, and then immersing substrates to be cleaned in said cleaning
solution for cleaning while allowing clean air to flow into the
cleaning draft, the apparatus for wet-cleaning a substrate
comprising: a clean air supplier for supplying the clean air into
the cleaning draft toward the level of the cleaning solution in the
cleaning bath downwards vertically; a humidifier for supplying
steam or mist -like water drops into the cleaning draft to maintain
the relative humidity of the clean air in the cleaning draft at a
value within a predetermined range; a hygrometer placed in the
neighborhood of the level of the cleaning solution in the cleaning
bath and connected to the humidifier; an exhaust piping for
allowing exhaust of air of the cleaning draft; and an exhaust rate
control means provided on said exhaust piping; wherein an amount of
steam or mist-like water drops supplied from the humidifier is
controlled within a predetermined range by feed-back of a humidity
value measured by the hygrometer to the humidifier; and air in the
cleaning draft is exhausted at the rate in a predetermined range by
said exhaust rate control means.
5. The apparatus for wet-cleaning a substrate according to claim 4,
wherein said clean air supplier comprises a blower and a dust
removal filter connected to the air discharge side of said blower,
said humidifier which supplies the steam or mist-like water drops
toward the level of the cleaning solution in the cleaning bath
downwards vertically is used, and said exhaust rate control means
comprises a damper having an automatically adjustable opening and
an airflow meter or a manometer provided on the exhaust piping,
while these meters are connected to the damper to control its
opening through feed-back of the value measured by the meters to
the damper.
6. The apparatus for wet-cleaning a substrate according to claim 4
or 5, further comprising: an air conditioner for controlling the
temperature of the clean air supplied toward the level of the
cleaning solution downwards vertically within a predetermined
range, wherein the humidifier is a unit as for supplying the steam
or mist-like water drops of a temperature controlled within a
predetermined range.
7. A method for cleaning a substrate with an aqueous solution of
ammonium fluoride as a cleaning solution, comprising the steps of:
measuring the concentration of a predetermined component in said
cleaning solution; and replenishing the cleaning solution with a
component for correcting the concentration of the predetermined
component when the result of measurement is out of a predetermined
range.
8. The method for cleaning a substrate according to claim 7,
wherein the component to be measured includes HF, and the cleaning
solution is replenished with the HF component when the result of
measurement is lower than the predetermined range, while the
cleaning solution is replenished with an ammonium component when
the result of measurement exceeds the predetermined range.
9. The method for cleaning a substrate according to claim 7,
wherein the cleaning solution is replaced after the lapse of a
predetermined period of time.
10. The method for cleaning a substrate according to claim 7,
wherein said concentration is measured every lapse of a
predetermined period of time.
11. An apparatus for cleaning a substrate with an aqueous solution
of ammonium fluoride as a cleaning solution, comprising: a
substrate cleaning bath stored with said cleaning solution; a
concentration measurement detector for measuring the concentration
of a predetermined component in said cleaning solution; and a
replenishing means for replenishing the cleaning solution with a
component for correcting the concentration of said predetermined
component when the result of measurement by said concentration
measurement detector is out of a predetermined range.
12. The apparatus for cleaning a substrate according to claim 11,
wherein the component to be measured includes HF, and the cleaning
solution is replenished with the HF component when the result of
measurement is lower than the predetermined range, while the
cleaning solution is replenished with an ammonium component when
the result of measurement exceeds the predetermined range.
13. The apparatus for cleaning a substrate according to claim 11,
further comprising: a solution replacement means for replacing the
cleaning solution after the lapse of a predetermined period of
time.
14. The apparatus for cleaning a substrate according to claim 11,
wherein said concentration measurement detector measures said
concentration every lapse of a predetermined time.
15. A method for wet-cleaning a substrate through the process of
successively immersing substrates to be cleaned in a plurality of
cleaning baths including at least a cleaning bath stored with a
cleaning solution containing an aqueous solution of ammonium
fluoride and/or hydrofluoric acid, the method for wet-cleaning a
substrate comprising the steps of: supplying a cleaning solution
from another cleaning bath to the cleaning bath stored with the
ammonium fluoride and/or hydrofluoric acid cleaning solution so as
to control the rise of hydrogen fluoride concentration of the
cleaning solution in said cleaning bath stored with the ammonium
fluoride and/or hydrofluoric acid cleaning solution.
16. The method for wet-cleaning a substrate according to claim 15,
wherein said another cleaning bath includes a bath stored with an
aqueous solution of ammonium with no content of fluoride.
17. The method for wet-cleaning a substrate according to claim 15,
wherein said another cleaning bath includes a bath stored with pure
water.
18. The method for wet-cleaning a substrate according to claim 15,
wherein said another cleaning bath includes a bath stored with
alcohol.
19. The method for wet-cleaning a substrate according to claim 18,
wherein said alcohol includes isopropyl alcohol.
20. A method for wet-cleaning a substrate through the process of
successively immersing substrates to be cleaned in a plurality of
cleaning baths including at least a cleaning bath stored with a
cleaning solution containing an aqueous solution of ammonium
fluoride and/or hydrofluoric acid, the method for wet-cleaning a
substrate, comprising the steps of: supplying a cleaning solution
from a cleaning bath in a different cleaning process to the
cleaning bath stored with the ammonium fluoride and/or hydrofluoric
acid cleaning solution so as to control the rise of hydrogen
fluoride concentration of the cleaning solution in said cleaning
bath stored with the ammonium fluoride and/or hydrofluoric acid
cleaning solution.
21. The method for wet-cleaning a substrate according to claim 20,
wherein the cleaning bath in said different cleaning process
includes a bath stored with an aqueous solution of ammonium with no
content of fluoride.
22. The method for wet-cleaning a substrate according to claim 20,
wherein the cleaning bath in said different cleaning process
includes a bath stored with pure water.
23. The method for wet-cleaning a substrate according to claim 20,
wherein the cleaning bath in said different cleaning process
includes a bath stored with alcohol.
24. The method for wet-cleaning a substrate according to claim 23,
wherein said alcohol includes isopropyl alcohol.
25. An apparatus for wet-cleaning a substrate, comprising: a
plurality of cleaning baths including at least a cleaning bath
stored with a cleaning solution containing an aqueous solution of
ammonium fluoride and/or hydrofluoric acid; and piping provided
between the cleaning bath stored with the ammonium fluoride and/or
hydrofluoric acid cleaning solution and another cleaning bath
stored with a solution for controlling the rise of hydrogen
fluoride concentration of the cleaning solution in said cleaning
bath stored with the ammonium fluoride and/or hydrofluoric acid
cleaning solution; wherein said another cleaning bath includes a
cleaning bath in the same cleaning process.
26. An apparatus for wet-cleaning a substrate, comprising: a
plurality of cleaning baths including at least a cleaning bath
stored with a cleaning solution containing an aqueous solution of
ammonium fluoride and/or hydrofluoric acid; and piping provided
between the cleaning bath stored with the ammonium fluoride and/or
hydrofluoric acid cleaning solution and another cleaning bath
stored with a solution for controlling the rise of hydrogen
fluoride concentration of the cleaning solution in said cleaning
bath stored with the ammonium fluoride and/or hydrofluoric acid
cleaning solution; wherein said another cleaning bath includes a
cleaning bath in a different cleaning process.
27. A method for cleaning a substrate with an aqueous solution of
ammonium fluoride at the ammonium fluoride (wt. %)-to hydrogen
fluoride (wt. %) ratio of >50 as a cleaning solution, comprising
the steps of: replenishing the cleaning solution with an ammonium
component with the lapse of time the cleaning solution is used.
28. The method for cleaning a substrate according to claim 27,
wherein the hydrogen fluoride concentration in said aqueous
solution is less than 1.0 wt. %.
29. An apparatus for cleaning a substrate with an aqueous solution
of ammonium fluoride at the ammonium fluoride (wt. %)-to-hydrogen
fluoride (wt. %) ratio of <50 as a cleaning solution,
comprising: a substrate cleaning bath stored with said cleaning
solution; and a replenishing means for replenishing the cleaning
solution in said substrate cleaning bath with water.
30. An apparatus for cleaning a substrate according to claim 29,
wherein the hydrogen fluoride concentration in said aqueous
solution is less than 1.0 wt. %.
31. A method for cleaning a substrate with an aqueous solution of
ammonium fluoride hydrofluoric acid mixture at the ammonium
fluoride (wt. %)-to-hydrogen fluoride (wt. %) ratio of .ltoreq.50
as a cleaning solution, comprising the step of: replenishing the
cleaning solution with water with the lapse of time said cleaning
solution is used.
32. The method for cleaning a substrate according to claim 31,
wherein the hydrogen fluoride concentration in said aqueous
solution is not less than 1.0 wt. %.
33. An apparatus for cleaning a substrate with an aqueous solution
of ammonium fluoride hydrofluoric acid mixture at the ammonium
fluoride (wt. %)-to-hydrogen fluoride (wt. %) ratio of .ltoreq.50
as a cleaning solution, comprising: a substrate cleaning bath
stored with said cleaning solution; and a replenishing means for
replenishing the cleaning solution in said substrate cleaning bath
with water.
34. The apparatus for cleaning a substrate according to claim 33,
wherein the hydrogen fluoride concentration in said aqueous
solution is not less than 1.0 wt. %.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Japanese Patent
Application No. JP2000-341094, JP2000-341093, JP2000-341092, JP
2000-363155, JP 2000-369632, and the disclosure of these
applications are incorporated herein by reference to the extent
permitted by law.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method and apparatus for
cleaning a substrate with an aqueous solution of ammonium fluoride
as a cleaning solution, and more specifically, to a novel substrate
cleaning method and apparatus developed with objectives of
providing a uniform and stable cleaning process with less amount of
cleaning solution consumed.
[0004] 2. Description of the Related Arts
[0005] Aqueous solutions of ammonium fluoride hydrofluoric acid
mixture and ammonium fluoride or hydrofluoric acid are used as a
cleaning solution for cleaning a semiconductor substrate, a glass
substrate in the process of manufacturing a semiconductor and a LCD
(Liquid Crystal Display). The semiconductor substrate is cleaned
mainly for the purpose of removing an oxide layer(a thermal oxide
layer or a native oxide layer) from the substrate surface by
etching, while the glass substrate is cleaned for the purpose of
removing a surface layer from the glass substrate similarly by
etching. Incidentally, the above aqueous solution of ammonium
fluoride is a desired hydrofluoric acid ammonium mixture solution,
which is dissolved in the form of NH.sub.4F in water and sometimes
contains a surfactant.
[0006] In the technical field of manufacture of the semiconductor
and the LCD, the art capable of finer processing is required for
higher integration to provide more lightweight, smaller-sized, less
power consumption products. In view of the above circumstances,
there has been a need for a more accurate cleaning process in the
substrate cleaning art using the above cleaning solution as
well.
[0007] Incidentally, wet-cleaning of the semiconductor substrate in
the related art is generally by the process of storing a cleaning
solution in a cleaning bath provided in a cleaning draft, loading a
number of semiconductor substrates in a container called a wafer
cassette, and then immersing the substrates inclusively of the
cassette in the cleaning solution for cleaning, while allowing
clean air to flow through the cleaning draft without the need for
special control.
[0008] In this related art, the HF (hydrofluoric acid)
concentration in the cleaning solution increases with the lapse of
cleaning time as shown in FIG. 1. Incidentally, FIG. 1 is a
graphical representation of the relation between the elapsed time
(cleaning time) since it started to use the cleaning solution for
the cleaning process and the HF concentration of the cleaning
solution. In consequence, the cleaning process in the above related
art is of no effect on uniform and stable cleaning by reason of the
considerably non-uniform change in concentration rise as well as
the increase in etching rate of the silicon oxide or the glass
substrate surface layer with the lapse of time as shown in FIG. 2,
resulting in problems inclusive of difficulty in providing higher
yield of the semiconductor substrate and the liquid crystal display
substrate.
[0009] The rise of HF concentration of the cleaning solution with
the lapse of cleaning time may occur due to the following causes.
That is, one of the causes is that a water content exhaled from the
cleaning solution is discharged to the outside of the cleaning
draft together with clean air. Another cause is that ammonium
hydroxide and hydrofluoric acid are dissociating from ammonium
fluoride (NH.sub.4F) in the aqueous solution of ammonium fluoride
so that ammonium hydroxide is discharged in the form of ammonium
gas to the outside of the cleaning draft similarly together with
the clean air.
[0010] For that reason, the rise of active component concentration
of the cleaning solution (inclusively of non-uniform concentration)
has been controlled by means of frequent replacement of the
cleaning solution in the related art. However, a considerably large
amount of cleaning solution is required for performing the above
measure. In particular, since the ammonium fluoride cleaning
solution is used at the concentration as high as about 40 wt. %,
there is a need for a larger amount of chemicals consumed per cycle
of solution replacement, as compared with other cleaning solutions
normally used at the concentration of about several wt. %.
[0011] A waste fluid treatment (a waste water treatment) according
to a process as shown in FIG. 3 is also required for the used
hydrofluoric acid cleaning solution (a hydrofluoric acid cleaning
solution 5). However, the waste fluid treatment in this case brings
about not only the need for a large amount of resources consumed,
that is, waste water treatment agents but also the increase in
released waste (waste water and sludge) amount with the increasing
amount of resources consumed, as is apparent from a material
balance view of FIG. 4. Incidentally, FIG. 3 is a flowsheet (a flow
of the treatment for an ammonium fluoride waste fluid released from
the semiconductor manufacture process) showing the process of the
waste fluid treatment for the used cleaning solution. FIG. 4 is a
view illustrating a material balance relating to the process of the
waste fluid treatment in FIG. 3.
[0012] Incidentally, the global environmental issues have been
recently made as a matter of worldwide concerns, and influences of
the semiconductor or LCD substrate manufacture process on the
environment have been at issue. In these days, there has been
socially a long-felt need for measures not only to decrease the
cleaning cost in the substrate manufacture process but also to meet
the preservation of the environment inclusive of the resource
saving, the decrease in released waste amount and the environmental
purification.
[0013] For that reason, an apparatus having a cleaning bath
installed under the atmosphere subjected to temperature, humidity
and clean air flow or like control has been proposed as a cleaning
apparatus structured to permit the chemical composition of a
cleaning solution in the cleaning bath to be maintained at a value
within a predetermined range without the need for frequent
replacement of the cleaning solution (See Japanese Patent Laid-open
No. 9-22891). However, this cleaning apparatus, although uses a
humidified air curtain, has difficulty in maintaining the
cleanliness of substrates to be cleaned by reason of the horizontal
flow of clean air, and besides, needs to discontinue the flow of
air every operation of taking in and out the substrates to be
cleaned, resulting in problems inclusive of the need for
troublesome operations.
[0014] As described the above, the substrate wet-cleaning process
in the related art is also of no effect on uniform and stable
cleaning by reason of the considerably non-uniform change in
concentration rise as well as the increase in etching rate of the
silicon oxide layer (or the glass substrate surface) with the lapse
of cleaning time (that is, with the elapsed time since it started
to use the cleaning solution for the cleaning process), resulting
in problems inclusive of difficulty in providing higher yield of
the semiconductor substrate and the liquid crystal display
substrate.
[0015] As the result of measurement conducted by the present
inventor as to the HF (hydrofluoric acid) concentration of the
above cleaning solution, it was ascertained that the HF
concentration of the cleaning solution increases with the lapse of
cleaning time as shown in FIG. 6, and the etching rate of the
silicon oxide increases with the increasing HF concentration of the
cleaning solution on the basis of the results shown in FIGS. 5 and
6, as shown in FIG. 7. It was also ascertained that the cleaning
apparatus having the cleaning bath installed in the cleaning draft
allowing the clean air to flow therethrough for exhaust of air
shows a particularly marked tendency toward the increase in etching
rate.
[0016] The rise of HF concentration of the cleaning solution with
the lapse of cleaning time occurs from the following causes That
is, one of the causes is that a water content is exhaled from the
cleaning solution. Another cause is that ammonium hydroxide and
hydrofluoric acid are dissociating from ammonium fluoride
(NH.sub.4F) in the aqueous solution of ammonium fluoride so that
the ammonium hydroxide is exhaled in the form of ammonium gas.
[0017] For that reason, the rise of active component concentration
of the cleaning solution (inclusively of the non-uniform
concentration) has been controlled by means of frequent replacement
of the cleaning solution. However, a considerably large amount of
cleaning solution is required for the above means. In particular,
since the ammonium fluoride cleaning solution is available at the
concentration as high as about 40 wt. %, there is a need for a
larger amount of ammonium fluoride and hydrofluoric acid consumed
per cycle of solution replacement, as compared with other cleaning
solutions normally available at the concentration of about several
wt. %.
[0018] A waste fluid treatment (a waste water treatment) according
to the process as shown in FIG. 3 is also required for the spent
hydrofluoric acid cleaning solution (the hydrofluoric acid cleaning
solution). However, the waste fluid treatment in this case brings
about not only the need for a large amount of resources consumed,
that is, waste water treatment agents but also the increase in
released waste (waste water 35, 45 and sludge 36, 46) amount with
the increasing amount of resources consumed, as is apparent from a
material balance view of FIG. 8. Incidentally, FIG. 8 is a view
illustrating a material balance relating to the process of the
waste fluid treatment in FIG. 3.
[0019] In Japanese Patent laid-open JP No. 9-22891 (the invention
titled "Apparatus and method for wet process"), there is disclosed
a wet process apparatus having an effect on a uniform
etching/cleaning process from the viewpoint of the lapse of time.
This wet process apparatus has two tanks, that is, a chemical
composition tank and a composition control tank, whereby
composition control chemicals are supplied from the composition
control tank to a chemical bath (an etching bath and a cleaning
bath) arranged at a predetermined location to maintain the
composition-changed chemicals in the chemical bath at the
predetermined composition.
[0020] However, the composition control tank is merely suitable for
storage of the composition control chemicals supplied to the
chemical bath, and the composition control chemicals are merely
available for composition control. The above disclosed apparatus is
fundamentally different in these points from the substrate
wet-cleaning method and apparatus according to the present
invention as will be described later, and shows the operations and
effects also quite different therefrom.
[0021] The process of cleaning or etching the substrate with the
aqueous solution of ammonium fluoride or ammonium fluoride
hydrofluoric acid mixture presents problems inclusive of the change
(increase) in etching rate for the silicon oxide or the glass
substrate by reason of the change (evaporation) in chemical
components (NH.sub.4F and HF) or water content in the cleaning
solution with the lapse of time.
[0022] The above problem is caused by the gradual increase in
concentration of hydrofluoric acid component in the cleaning
solution with the lapse of time in the process of being used. On
the other hand, in the existing circumstances, frequent replacement
of the cleaning solution has been applied as a measure to cope with
the concentration change in various components in the cleaning
solution.
[0023] However, a considerably large amount of cleaning solution is
required for the above measure (In particular, since the ammonium
fluoride cleaning solution is used at the concentration as high as
several ten wt. % (about 40 wt. %, for instance), there is a need
for a larger amount of chemicals consumed per cycle of solution
replacement, as compared with other cleaning solutions normally
available at the concentration of about several wt. %.), resulting
in the increase in ammonium fluoride or hydrofluoric acid
consumption (chemical cost) with the increasing amount of cleaning
solution used.
[0024] The waste water treatment as shown in FIG. 3 is also
required for the used cleaning solution. However, the waste water
treatment in this case brings about not only the need for a large
amount of resources (waste water treatment agents) consumed but
also the increase in released waste (waste water and sludge) amount
with the increasing amount of resources consumed (See FIG. 4).
[0025] FIG. 3 is a view illustrating the process of the waste fluid
treatment for the used cleaning solution. As shown in FIG. 3, the
waste fluid discharged from the cleaning bath 1 is carried to a pH
control tank 2 for neutralization with a 20% calcium hydroxide
solution. Subsequently, the neutralized waste fluid is carried to a
coagulo-sedimentation tank 3 for coagulo-sedimentation with
chemicals such as aluminum sulfate, and is further carried to a
coagulation tank 4 for coagulation and precipitation with a
polymeric coagulant such as a polyacrylic amide coagulant, and
finally, the resultant is released as sewage and sludge.
[0026] In the above process of the waste fluid treatment as shown
in FIG. 4, 2.0 Kg of 20% calcium hydroxide, 0.3 Kg of 8% aluminum
sulfate and 1.6 Kg of polymeric coagulant are used for 1.0 Kg of an
aqueous solution of 40% ammonium fluoride, and 2.6 Kg of 70% solid
sludge and 2.3 Kg of waste water are released as wastes.
[0027] Incidentally, the global environmental issues have been
recently made as a matter of worldwide concerns, and the
environmental burden on the semiconductor or LCD substrate
manufacture process has been at issue. In these days, there has
been socially a long-felt need also on the cleaning solution for
measures not only to decrease the cost but also to meet the
preservation of the environment inclusive of the resource saving,
the decrease in released waste amount and the environmental
purification.
SUMMARY OF THE INVENTION
[0028] A first object of the present invention is to, in the
substrate wet-cleaning art using a cleaning solution prepared by
dissolving hydrofluoric acid as an active component in water,
realize a decrease in environmental burden resulting from the use
of the cleaning solution, inclusive of a decrease in cleaning
chemical consumption, a decrease in released waste amount and a
control of environmental pollution and so on, while permitting a
cleaning process to be made uniform and stable.
[0029] A second object of the present invention is to, in a method
and apparatus for cleaning a substrate through the process of
successively immersing substrates to be cleaned in a plurality of
cleaning baths including at least a cleaning bath stored with a
cleaning solution containing an aqueous solution of ammonium
fluoride and/or hydrofluoric acid, realize a decrease in
environmental burden resulting from the use of the cleaning
solution, inclusive of a decrease in cleaning chemical consumption,
a decrease in released waste amount and a control of environmental
pollution and so on, while permitting a cleaning process to be made
uniform and stable by controlling the HF concentration rise
occurring in the cleaning solution with the lapse of cleaning
time.
[0030] A third object of the present invention is to provide a
substrate cleaning method and a substrate cleaning apparatus, which
have an effect of making a substrate cleaning process uniform and
stable, and also permit a decrease in released waste amount and so
on with the decreasing amount of resources consumed.
[0031] In a method for cleaning a substrate through the process of
storing a cleaning solution prepared by dissolving active
components in water in a cleaning bath provided in a cleaning
draft, and then immersing substrates to be cleaned in the cleaning
solution for cleaning while allowing clean air to flow into the
cleaning draft, a method for wet-cleaning a substrate according to
an embodiment of the present invention comprises the steps of
cleaning the substrates to be cleaned, while supplying clean air
humidified to control the relative humidity within a predetermined
range toward the level of the cleaning solution in the cleaning
bath downwards vertically, together with exhaust of air of the
cleaning draft at an exhaust rate in a predetermined range.
[0032] According to the cleaning method described the above, the
clean air supplied is preferably limited in relative humidity to
the range of 40 to 50% and also in temperature to a room
temperature, providing more increased effects of the present
invention. Atmospheric pressure in the cleaning draft is also
preferably limited to a value slightly higher than that outside the
cleaning draft for preventing the air outside the cleaning draft
from entering the cleaning draft. With this, the cleanliness of
clean air in the cleaning draft is maintained, and the flow-through
state, temperature and humidity of the clean air are stabilized,
permitting the continuous operation of more stable wet
cleaning.
[0033] According to the cleaning method described the above, a
silicon oxide or a surface layer may be removed from a silicon
substrate or a glass substrate efficiently by etching by use of the
solution prepared by dissolving at least one of ammonium fluoride
and hydrofluoric acid in water as the cleaning solution.
[0034] Also, in an apparatus for cleaning a substrate through the
process of storing a cleaning solution prepared by dissolving
active components in water in a cleaning bath provided in a
cleaning draft, and then immersing substrates to be cleaned in the
cleaning solution for cleaning while allowing clean air to flow
into the cleaning draft, an apparatus for wet-cleaning a substrate
according to an embodiment of the present invention comprises a
clean air supplier for supplying the clean air into the cleaning
draft toward the level of the cleaning solution in the cleaning
bath downwards vertically, a humidifier for supplying steam or
mist-like water drops into the cleaning draft to maintain the
relative humidity of the clean air in the cleaning draft at a value
within a predetermined range, a hygrometer placed in the
neighborhood of the level of the cleaning solution in the cleaning
bath and connected to the humidifier, an exhaust piping for
allowing exhaust of air of the cleaning draft, and an exhaust rate
controller provided on the exhaust piping, wherein an amount of
steam or mist-like water drops supplied from the humidifier is
controlled within a predetermined range by feed-back of a humidity
value measured by the hygrometer to the humidifier, and air of the
cleaning draft is exhausted at the rate within a predetermined
range under controlling of the exhaust rate controller.
[0035] According to the wet-cleaning apparatus, preferably, the
clean air supplier comprises a blower and a dust removal filter
connected to the air discharge side of the blower, the humidifier
for supplying the steam or mist-like water drops toward the level
of the cleaning solution in the cleaning bath downwards vertically
is used, and the exhaust rate controller may comprise a damper
having an automatically adjustable opening and an airflow meter or
a manometer provided on the exhaust piping, while these meters are
connected to the damper to control the opening of the damper by
feed-back of values measured by these meters to the damper.
[0036] Preferably, the wet-cleaning apparatus further comprises a
unit provided as the humidifier for supplying steam or mist-like
water drops of a temperature controlled within a predetermined
range and an air conditioner for controlling the temperature of the
clean air supplied toward the level of the cleaning solution
downwards vertically within the predetermined range.
[0037] The substrate cleaning method and apparatus according to an
embodiment of the present invention are realized on the basis of
the fact discovered by the present inventor that the rise of HF
concentration with the lapse of time is controlled with a cleaning
solution supplied from another cleaning bath or from a cleaning
bath in a different cleaning process, thereby providing a uniform
and stable substrate cleaning process (a uniform and stable etching
rate). According to the present inventor, it was also ascertained
that using the above method for cleaning the substrate permits the
decrease in frequency in replacement of the cleaning solution, the
decrease in cleaning chemical consumption and the decrease in
released waste amount with the decreasing amount of chemicals
consumed for the waste fluid treatment and so on.
[0038] That is, in a method for cleaning a substrate through the
process of successively immersing substrates to be cleaned in a
plurality of cleaning baths including at least a cleaning bath
stored with a cleaning solution containing an aqueous solution of
ammonium fluoride and/or hydrofluoric acid, a method for
wet-cleaning a substrate according to an embodiment of the present
invention comprises the steps of supplying a cleaning solution from
another cleaning bath to the cleaning bath stored with the ammonium
fluoride and/or hydrofluoric acid cleaning solution to control the
rise of HF concentration of the cleaning solution in the above
cleaning bath stored with the ammonium fluoride and/or hydrofluoric
acid cleaning solution.
[0039] The method for wet-cleaning a substrate according to the
present embodiment is also characterized in that another cleaning
bath described the above is a bath stored with an aqueous solution
of ammonium with no content of fluoride. Another cleaning bath
described the above may be also a bath stored with pure water or a
bath stored with alcohol. More preferably, the alcohol used may be
isopropyl alcohol.
[0040] Also, in a method for cleaning a substrate through the
process of successively immersing substrates to be cleaned in a
plurality of cleaning baths including at least a cleaning bath
stored with a cleaning solution containing an aqueous solution of
ammonium fluoride and/or hydrofluoric acid, a method for
wet-cleaning a substrate according to an embodiment of the present
invention comprises the steps of supplying a cleaning solution from
a cleaning bath in a different cleaning process to the cleaning
bath stored with the ammonium fluoride and/or hydrofluoric acid
cleaning solution to control the rise of HF concentration of the
cleaning solution in the above cleaning bath stored with the
ammonium fluoride and/or hydrofluoric acid cleaning solution.
[0041] The method for wet-cleaning a substrate according to the
present embodiment is also characterized in that the cleaning bath
in the different cleaning process is a bath stored with an aqueous
solution of ammonium with no content of fluoride. The cleaning bath
in the different cleaning process may also be a bath stored with
pure water or a bath stored with alcohol. More preferably, the
alcohol used may be isopropyl alcohol.
[0042] Also, an apparatus for wet-cleaning a substrate according to
the second object of the present invention comprises a plurality of
cleaning baths including at least a cleaning bath stored with a
cleaning solution containing an aqueous solution of ammonium
fluoride and/or hydrofluoric acid, and piping provided between the
cleaning bath stored with the ammonium fluoride and/or hydrofluoric
acid cleaning solution and another cleaning bath stored with a
solution for controlling the rise of HF concentration of the
cleaning solution in the cleaning bath stored with the ammonium
fluoride and/or hydrofluoric acid cleaning solution, wherein
another cleaning bath is a cleaning bath in the same cleaning
process.
[0043] Also, an apparatus for wet-cleaning a substrate according to
an embodiment of the present invention comprises a plurality of
cleaning baths including at least a cleaning bath stored with a
cleaning solution containing an aqueous solution of ammonium
fluoride and/or hydrofluoric acid, and piping provided between the
cleaning bath stored with the ammonium fluoride and/or hydrofluoric
acid cleaning solution and another cleaning bath stored with a
solution f or controlling the rise of HF concentration of the
cleaning solution in the cleaning bath stored with the ammonium
fluoride and/or hydrofluoric acid cleaning solution, wherein
another cleaning bath is a cleaning bath in a different cleaning
process.
[0044] As described the above, the method and apparatus for
wet-cleaning the substrate according to the present embodiment are
characterized in that the rise of HF concentration of the cleaning
solution in the cleaning bath for cleaning the substrate after
being immersed in the cleaning solution is controlled by means of
diluting the cleaning solution with (1) the cleaning solution
supplied from another cleaning bath installed together with the
cleaning bath to constitute a series of cleaning processes or (2)
the cleaning solution supplied from a cleaning bath constituting a
cleaning process different from the above series of cleaning
processes. That is, although either of the cleaning solution in
each of the plurality of cleaning baths constituting the series of
cleaning processes or the cleaning solution in the cleaning bath
constituting the different cleaning process is treated as wastes in
the process of the waste water treatment whenever a predetermined
period of cleaning time has elapsed, the method and apparatus of
the present embodiment permit the effective utilization of the
above cleaning solutions as diluent, thus providing a uniform and
stable substrate cleaning process.
[0045] A method and apparatus according to an embodiment of the
present invention are provided in view of the presence of problems
with a cleaning or etching process, inclusive of the change
(increase) in etching rate for the silicon oxide or glass substrate
by reason of the change (evaporation) in chemical components
(NH.sub.4F, HF) or water content in the cleaning solution with the
lapse of time.
[0046] The above problem is caused by the gradual increase in
concentration of hydrofluoric acid component in the cleaning
solution with the lapse of time in the process of being used. On
the other hand, in the existing circumstances, frequent replacement
of the cleaning solution has been applied as a measure to cope with
the change in concentration of various components in the cleaning
solution.
[0047] However, a considerably large amount of cleaning solution is
required for the above measure. In particular, since the ammonium
fluoride cleaning solution is available at the concentration as
high as several ten wt. % (about 40 wt. %, for instance).
Accordingly, there is a need for a larger amount of chemicals
consumed per cycle of solution replacement, as compared with other
cleaning solutions normally used at the concentration of about
several wt. %., resulting in the increase in ammonium fluoride or
hydrofluoric acid consumption (cost for chemical products) with the
increasing amount of cleaning solution used.
[0048] The waste water treatment as shown in FIG. 3 is also
required for the used cleaning solution. The waste water treatment
in this case brings about not only the need for a large amount of
resources (waste water treatment agents) consumed but also the
increase in released waste (waste water and sludge) amount with the
increasing amount of resources consumed (See FIG. 8).
[0049] FIG. 3 is a view illustrating the process of the waste fluid
treatment for the cleaning solution. As shown in FIG. 3, the waste
fluid discharged from the cleaning bath 1 is carried to a pH
control tank 2 for neutralization with a 20% calcium hydroxide
solution, for instance. Subsequently, the neutralized waste fluid
is carried to a coagulo-sedimentation tank 3 for
coagulo-sedimentation with chemicals such as aluminum sulfate, and
is then carried to a coagulation tank 4 for coagulation and
precipitation with a polymeric coagulant such as a polyacrylic
amide coagulant, and finally, the resultant is released as sewage
and sludge.
[0050] In the process of the waste fluid treatment as shown in FIG.
8, 2.0 Kg of 20% calcium hydroxide, 0.3 Kg of 8% aluminum sulfate
and 1.6 Kg of polymeric coagulant are used for 1.0 Kg of an aqueous
solution of 40% ammonium fluoride, and 2.6 Kg of 70% solid sludge
and 2.3 Kg of waste water are released as wastes.
[0051] The present embodiment is also provided in view of the above
circumstances, and there is provided a substrate cleaning method
and a substrate cleaning apparatus, which have an effect of making
a cleaning process uniform and stable and also permit the decrease
in released waste amount and so on with the decreasing amount of
resources consumed.
[0052] In a method for cleaning a substrate with an aqueous
solution of ammonium fluoride at the ammonium fluoride (wt.
%)-to-hydrogen fluoride (wt. %) ratio of more than 50 (>50) as a
cleaning solution, a method for cleaning a substrate according to
an embodiment of the present invention comprises the step of
replenishing the cleaning solution with an ammonium component with
the lapse of time the cleaning solution was used.
[0053] In an apparatus for cleaning a substrate with an aqueous
solution of ammonium fluoride at the ammonium fluoride (wt.
%)-to-hydrogen fluoride (wt. %) ratio of more than 50 (>50) as a
cleaning solution, an apparatus for cleaning a substrate according
to an embodiment of the present invention comprises a substrate
cleaning bath stored with the cleaning solution and a replenishing
means for replenishing the cleaning solution in the substrate
cleaning bath with an ammonium component.
[0054] When the aqueous solution of ammonium fluoride or ammonium
fluoride hydrogen fluoride mixture is used as the cleaning solution
for cleaning or etching the substrates, the concentration and
composition of the cleaning solution change with the lapse of
time.
[0055] According to the experiment conducted by the present
inventor, it is found that the state of the change in concentration
and composition varies depending on the ammonium
fluoride-to-hydrogen fluoride ratio when the aqueous solution of
ammonium fluoride hydrogen fluoride mixture is used as the cleaning
solution.
[0056] When the composition ratio of ammonium fluoride (wt. %) to
hydrogen fluoride (wt. %) is more than 50 (>50), for instance,
the change in ammonium fluoride-to-hydrogen fluoride ratio occurs
(lowers) with the lapse of time. As a result, the hydrogen fluoride
rate in the cleaning solution is increased.
[0057] On the other hand, when the composition ratio of ammonium
fluoride (wt. %) to hydrogen fluoride (wt. %) is not more than 50
(.ltoreq.50), for instance, the change in ammonium
fluoride-to-hydrogen fluoride ratio hardly occurs.
[0058] In other words, when the aqueous solution having the
ammonium fluoride (wt. %)-to-hydrogen fluoride (wt. %) ratio of not
more than 50 (.ltoreq.50) is used as the cleaning solution for
cleaning the substrates, the process of replenishing the above
cleaning solution with water is required with the lapse of time
while the cleaning solution is being used.
[0059] On the other hand, when the aqueous solution having the
ammonium fluoride (wt. %)-to-hydrogen fluoride (wt. %) ratio of
more than 50 (>50) is used as the cleaning solution for cleaning
the substrates, the process of replenishing the above cleaning
solution with an ammonium component is required with the lapse of
time while the cleaning solution is being used.
[0060] As described the above, when the composition ratio of
ammonium fluoride (wt. %) to hydrogen fluoride (wt. %) is more than
50 (>50), the gradual change in ammonium fluoride-to-hydrogen
fluoride ratio occurs with the lapse of time. However, excessive
hydrogen fluoride is converted into ammonium fluoride by means of
replenishing the cleaning solution with the ammonium component, and
as a result, the HF concentration is maintained constant.
[0061] With the above process, the amount of cleaning solution
consumed is decreased with the decreasing frequency in replacement
of the cleaning solution, permitting the decrease in amount of
waste water and sludge released from the waste water treatment with
the decreasing amount of chemicals for the waste water treatment
for the cleaning solution.
[0062] That is, according to the present embodiment, the substrate
cleaning process is made uniform and stable, and besides, the
amount of wastes released is decreased with the decreasing amount
of cleaning solution consumed.
[0063] Also, in an apparatus for cleaning a substrate with an
aqueous solution of ammonium fluoride and hydrogen fluoride at the
ammonium fluoride (wt. %)-to-hydrogen fluoride (wt. %) ratio of not
more than 50 (.ltoreq.50), an apparatus for cleaning a substrate
according to an embodiment of the present invention comprises a
substrate cleaning bath stored with the cleaning solution and a
replenishing means for replenishing the cleaning solution in the
substrate cleaning bath with water.
[0064] When the aqueous solution of ammonium fluoride hydrogen
fluoride mixture is used as the cleaning solution for cleaning or
etching the substrates, the concentration and composition of the
cleaning solution change with the lapse of time.
[0065] According to the experiment conducted by the present
inventor, it is found that the state of the change in concentration
and composition varies depending on the ammonium
fluoride-to-hydrogen fluoride ratio when the aqueous solution of
ammonium fluoride hydrogen fluoride mixture is used as the cleaning
solution.
[0066] When the composition ratio of ammonium fluoride (wt. %) to
hydrogen fluoride (wt. %) is not more than 50 (.ltoreq.50), for
instance, the change in ammonium fluoride-to-hydrogen fluoride
ratio hardly occurs.
[0067] In this connection, according to the present embodiment,
when the aqueous solution having the ammonium fluoride (wt.
%)-to-hydrogen fluoride (wt. %) ratio of not more than 50
(.ltoreq.50) is used as the cleaning solution for cleaning the
substrates, the process of replenishing the cleaning solution with
water is required with the lapse of time while the cleaning
solution is being used.
[0068] Since the change in ammonium fluoride to hydrogen fluoride
ratio hardly occurs when the composition ratio of ammonium fluoride
(wt. %) to hydrogen fluoride (wt. %) is not more than 50
(.ltoreq.50) as described the above, replenishment of the cleaning
solution with water will be only enough to maintain the
concentration constant.
[0069] With this, the amount of cleaning solution consumed is
decreased with the decreasing frequency in replacement of the
cleaning solution, permitting the decrease in amount of waste water
and sludge released from the waste water treatment with the
decreasing amount of chemicals f or the waste water treatment for
the cleaning solution.
[0070] That is, according to the present embodiment of the present
invention, the substrate cleaning process is made uniform and
stable, and besides, the amount of wastes released is decreased
with the decreasing amount of cleaning solution consumed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0071] The foregoing and other objects and features of the
invention will become apparent from the following description of
preferred embodiments of the invention with reference to the
accompanying drawings, in which:
[0072] FIG. 1 is a graphical representation of the relation between
the elapsed time since it started to use a cleaning solution for a
cleaning process and the HF (hydrofluoric acid) concentration of
the cleaning solution in accordance with the problems with a
cleaning method in the related art;
[0073] FIG. 2 is a graphical representation of the relation between
the lapse of cleaning time and the etching rate of a semiconductor
substrate when the cleaning solution described in FIG. 2 is
used;
[0074] FIG. 3 is a flow sheet showing a process of the waste fluid
treatment for the used cleaning solution, that is, the treatment of
aluminum fluoride waste fluid released from a semiconductor
manufacture process in accordance with the problems with the
cleaning method in the related art;
[0075] FIG. 4 is a schematic illustration of a material balance
relating to the process of the waste fluid treatment in FIG. 3;
[0076] FIG. 5 is a graphical representation of the relation between
the elapsed time (cleaning time) since it started to use a cleaning
solution for a cleaning process and the etching rate of a silicon
oxide on the surface of a semiconductor substrate in accordance
with the problems with the cleaning method in the related art;
[0077] FIG. 6 is a graphical representation of the relation between
the elapsed time since it started to use a cleaning solution for a
cleaning process and the HF concentration of a cleaning solution
when the cleaning solution described in FIG. 5 is used;
[0078] FIG. 7 is a graphical representation of the relation between
the HF concentration of a cleaning solution and the etching rate of
a silicon oxide on the surface of a semiconductor substrate when
the cleaning solution described in FIG. 6 is used;
[0079] FIG. 8 is a schematic illustration of a material balance
relating to the process of the waste fluid treatment;
[0080] FIG. 9 is a schematic illustration of a structure of a
wet-cleaning apparatus according to an embodiment of the present
invention, together with a substrate cleaning method carried into
operation using this wet-cleaning apparatus;
[0081] FIG. 10 is a graphical representation of the etching rate of
a silicon oxide on the surface of a semiconductor substrate when
the cleaning method of the present invention is used, in comparison
with that when the cleaning method in the related art is used;
[0082] FIG. 11 is a characteristic graphical representation of the
relation in a cleaning solution between the elapsed time and the
etching rate;
[0083] FIG. 12 is a characteristic graphical representation of the
relation between the elapsed time and the HF concentration of the
cleaning solution;
[0084] FIG. 13 is a flow chart showing the process flow of a
cleaning system according to the present invention;
[0085] FIGS. 14 to 18 are schematic illustrations of a structure of
a wet-cleaning apparatus according to embodiments of the present
invention, together with a substrate cleaning method carried into
operation using this wet-cleaning apparatus, respectively;
[0086] FIG. 19 is a graphical representation of the change in
etching rate for a silicon oxide on the surface of a semiconductor
substrate and the change in HF concentration of the cleaning
solution when the cleaning method of the present invention is used,
in comparison with those when the cleaning method in the related
art is used;
[0087] FIG. 20 is a characteristic graphical representation of the
relation in a cleaning solution between the elapsed time and the
etching rate;
[0088] FIG. 21 is a characteristic graphical representation of the
relation between the elapsed time and the HF concentration of the
cleaning solution;
[0089] FIG. 22 is a characteristic graphical representation of the
change in NH.sub.4F-to-HF ratio depending on the lapse of time;
[0090] FIG. 23 is a schematic illustration of an apparatus for
cleaning a substrate according to one embodiment of the present
invention;
[0091] FIG. 24 is a graphical representation of the controlled
state of the etching rate and the HF concentration depending on
replenishment with an ammonium component;
[0092] FIG. 25 is a schematic illustration of an apparatus for
cleaning a substrate according to another embodiment of the present
invention;
[0093] FIG. 26 is a characteristic graphical representation of the
relation in a cleaning solution between the elapsed time and the
etching rate;
[0094] FIG. 27 is a characteristic graphical representation of the
relation between the elapsed time and the HF concentration of the
cleaning solution;
[0095] FIG. 28 is a characteristic graphical representation of the
change in NH.sub.4F-to-HF ratio depending on the lapse of time;
[0096] FIG. 29 is a schematic illustration of an apparatus for
cleaning a substrate according to a further embodiment of the
present invention;
[0097] FIG. 30 is a characteristic graphical representation of the
change in HF concentration depending on replenishment with
water;
[0098] FIG. 31 is a characteristic graphical representation of the
change in etching rate depending on replenishment with water;
[0099] FIG. 32 is a graphical representation of the controlled
state of the etching rate and the HF concentration depending on
replenishment with water; and
[0100] FIG. 33 is a schematic illustration of an apparatus for
cleaning a substrate according to a still further embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0101] FIG. 9 is a schematic illustration of an apparatus for
cleaning a substrate (a wet-cleaning apparatus), together with a
substrate cleaning method carried into operation using this
wet-cleaning apparatus. FIG. 10 is a graphical representation of
the etching rate of a silicon oxide on the surface of a
semiconductor substrate when a cleaning method of the present
invention is used, in comparison with that when the cleaning method
in the related art is used.
[0102] A substrate cleaning apparatus 100 has the following
structure. A cleaning bath 112 is provided at a lower part of a
cleaning draft (a draft chamber) 111 having a pivotal door (not
shown), and a hygrometer (or any type of detector measuring
humidity) 113 is installed in the vicinity (of the surface level of
a cleaning solution 131) of an upper end of the cleaning bath 112.
A humidifier 114 and an air conditioner 115 are provided right
above the cleaning bath 112, and a dust removal filter 115 is
connected to the air inflow side of the air conditioner 115, while
the hygrometer 113 is connected to the humidifier 114. The air
inflow side of the dust removal filter 116 is connected to the air
discharge side of a blower through an air supply duct, although not
shown. A temperature control means (not shown) for controlling the
solution temperature rise occurring with the lapse of cleaning time
to maintain a cleaning solution at a room temperature is preferably
provided in the cleaning bath 112.
[0103] An exhaust duct 117 is connected to a portion of a
peripheral wall 111a of the cleaning draft 111 as being close to
the upper end of the cleaning bath 112. The exhaust duct 117 has an
automatic damper 118 and an airflow meter 119 (or a manometer) at
the downstream side of the automatic damper. The airflow meter 119
may be either at the upstream side of the automatic damper 118 or
in the cleaning draft 111.
[0104] Although only the cleaning bath 112 (an etching bath) is
shown in the cleaning draft 111 in FIG. 9, a rinsing bath and a
drying bath (not shown) may be also disposed at locations adjacent
to the cleaning bath in the cleaning draft 111. In this case, the
rinsing bath which may be for immersing the substrate in
ion-exchange water, for instance, is used, and the drying bath
which may be for drying the substrate by blowing clean air
thereagainst after spraying the substrate surface with IPA
(isopropyl-alcohol), for instance, is used. Incidentally, the
cleaning draft 111, the rinsing bath and the drying bath, which are
installed in this order individually in partition chambers
partitioned with walls in such a way that the adjacent partition
chambers are allowed to communicate with each other by opening
pivotal doors provided in the walls, will be also enough.
[0105] A description will now be given of a method for cleaning a
silicon substrate using the substrate cleaning apparatus 100. A
predetermined amount of room-temperature cleaning solution 131
prepared by dissolving ammonium fluoride and hydrofluoric acid in
water to meet predetermined concentrations is supplied into the
cleaning bath 112. After the automatic damper 118 is opened at an
appropriate opening, power to the humidifier 114 and the air
conditioner 115 is turned ON. Since the cleaning draft 111 needs to
have the ambient with a relative humidity in which further
evaporation of water may be prevented so as to control the
evaporation loss of water from the cleaning solution 131, an
objective value (preferably, a value close to saturation humidity)
of the relative humidity is set by the humidifier 114. An objective
value of the temperature is also set by the air conditioner 115 for
limiting the atmosphere in the cleaning draft 111 to a
predetermined temperature. An objective value of the exhaust rate
is also set with respect to a drive mechanism of the automatic
damper 118 for controlling the exhaust rate of air (per unit time)
in the cleaning draft 111.
[0106] A number of silicon substrates (silicon wafers) are loaded
in a wafer cassette in such a way that the substrates are placed
vertically in parallel to each other with proper spaces in between.
Then, the silicon substrates inclusive of the wafer cassette are
immersed in the cleaning solution 131 to start the cleaning
process. The silicon substrates in this cleaning process are
cleaned in such a way that a native oxide layer is removed from the
surface of each silicon substrate by this etching process.
[0107] In the cleaning process, air (limited in temperature to
approximately a room temperature and in pressure to a value higher
than the atmospheric pressure) is supplied from the blower to the
substantially whole area of the surface level of the cleaning
solution 131 in the cleaning bath 112 downwards vertically through
the dust removal filter 116, the air conditioner 115 and the
humidifier 114. In this case, the air from the blower is cleaned to
a high degree with the dust removal filter 116, and the temperature
of air is controlled within a predetermined range (normally, at
substantially a room temperature) with the air conditioner 115. The
relative humidity of air is also controlled within a predetermined
range with mist-like pure water drops sprayed from the humidifier
114. Clean air subjected to contact with the level of the cleaning
solution is exhausted to the outside through the automatic damper
118 at a predetermined flow rate.
[0108] According to the cleaning process as described the above,
the humidified clean air limited in pressure to a value higher than
the atmospheric pressure and in temperature and humidity to
predetermined values is supplied toward the substantial whole level
of the cleaning solution 131 in the cleaning bath 112 downwards
vertically. Thus, as compared with the process of allowing the
clean air to flow in parallel to he level of the cleaning solution,
the above cleaning process enhances substantially a function of
controlling the evaporation loss of water from the cleaning
solution, and also provides a higher function of air displacement
at the level of the cleaning solution. In addition, the above
cleaning process offers the advantages of more efficient
displacement of air in spaces between the wafers with clean air
when handling the wafer cassette loaded with the silicon
wafers.
[0109] According to the above cleaning process, since the
atmospheric pressure in the cleaning draft 111 is limited to a
value slightly higher than the atmospheric pressure outside the
cleaning draft, air outside the cleaning draft 111 may be prevented
from entering the cleaning draft 111. With this arrangement, the
above cleaning process eliminates the defects inclusive of the
pollution of clean air in the cleaning draft due to the outside air
entering the cleaning draft as well as the variations in
flow-through state, temperature and humidity of the clean air in
the cleaning draft, permitting the continuous operation of more
stable wet-cleaning.
[0110] In an example of the present embodiment in accordance with
the present invention, the cleaning apparatus shown in FIG. 9 is
used to clean the silicon wafers according to the above cleaning
method. On the other hand, in a comparative example, the silicon
wafers were cleaned similarly to the example, except that the
cleaning apparatus 100 of FIG. 9 was used without operating the air
conditioner 115 and the humidifier 114 and the opening of the
automatic damper 118 was held constant. The results are shown in
FIG. 10.
[0111] According to the result of the example, the rise of etching
rate with the elapsed time since the cleaning process is occurred
slowly, and besides, the substantially constant rise of etching
rate is observed as shown in FIG. 10. For that reason, there was no
need for solution replacement, regardless of the long-term use of
the same cleaning solution On the other hand, according to the
result of the comparative example, the rise of etching rate to the
elapsed time since the cleaning process is started occurred at a
rate higher than that in the example, and besides, the non-uniform
change in etching rate rise is observed. Thus, the solution was
replaced in the course of the cleaning process. However, cleaning
in a short period of time after the replacement of cleaning
solution caused the rise of etching rate, and the etching rate
became soon higher than that using the method according to the
example requiring no replacement of cleaning solution.
[0112] The humidifier which utilizes ultrasonic waves may be used
for the cleaning apparatus of FIG. 9, or supply of steam from
boiling water into the cleaning draft for humidifying is also
applicable. An operation of supplying the clean air limited in
temperature and humidity as described the above at a predetermined
flow rate may be also effectively applied to the case of
replenishing the cleaning solution with various components
inclusive of ammonium fluoride, hydrofluoric acid, aqueous
ammonium, water and a surfactant.
[0113] Measurement on the concentration of active components and
water in the cleaning solution may be performed by employing
methods for measuring (1) the absorbance of light with a
predetermined wavelength, (2) infrared/ultraviolet absorption
spectrum, and (3) refractive index, specific gravity, light
transmittance and conductivity or the like, or using a measuring
unit such as (4) a Karl Fischer's titration meter and (5) a liquid
(ion) chromatography or the like.
[0114] Using the above cleaning method may lower substantially the
change or non-uniformity in etching rate, that is, cleaning effects
with the lapse of cleaning time as to the cleaning solution
prepared by dissolving at least hydrofluoric acid as an active
component in water, as shown in FIG. 10. Accordingly, the cleaning
process with the above cleaning solution may be made uniform and
stable, permitting the substantial decrease in amount of chemicals
for the waste fluid treatment (the waste water treatment) for the
above cleaning solution with the decreasing frequency in solution
replacement (the extended life of the cleaning solution). The above
cleaning method also has an effect of decreasing the amount of
wastes released, that is, sludge and waste water (sewage) with the
decreasing amount of chemicals consumed.
[0115] According to the cleaning method of the present embodiment,
since the fixed linear relation is observed between the elapsed
time since the cleaning process is started and the etching rate as
shown in FIG. 10, the etching rate (a value of a vertical axis when
the elapsed time is given by 0 in FIG. 2) at the starting time of
the cleaning process is determined depending on the HF
(hydrofluoric acid) concentration of the cleaning solution at the
starting time of the cleaning process, while the etching rate after
start of the cleaning process is automatically determined depending
on the elapsed time since the cleaning process was started.
[0116] Accordingly, the HF concentration of the cleaning solution
at the starting time of the cleaning process and the elapsed time
since the cleaning process is started are required to estimate the
etching rate simply. The above cleaning method further has an
effect on highly reproducible cleaning enough to permit cleaning at
a uniform and constant etching rate. On the other hand, the
required etching time (the above elapsed time since the cleaning
process is started) is simply obtained by setting the required
etching rate. Furthermore, the required etching time or the HF
concentration of the required cleaning solution may be obtained
more simply by formulating the above linear relation into a
numerical expression.
[0117] Incidentally, in Japanese Patent Laid-open No. 9-22891
(relating to the invention titled "Apparatus and method for
wet-cleaning process"), there is disclosed an apparatus for etching
and cleaning an oxide from the surface of a silicon wafer. In the
apparatus, a cleaning bath stored with a chemical solution is
installed under the atmosphere limited in temperature, humidity and
clean air flow rate for cleaning wafers while allowing clean air
limited in relative humidity to 70% or more to flow horizontally
toward an upper part of the level of the chemical solution in the
cleaning bath.
[0118] However, according to the above apparatus, the clean air,
when being allowed to continuously flow, is blown vertically
against the wafer surface also for the duration of the operation of
pulling up the wafer cassette from the cleaning bath after cleaning
for transport to the next cleaning bath, for instance, resulting in
a turbulent flow of clean air on the wafer surface. Thus, adhesion
of so-called particles (foreign matters) to the cleaned wafer
surface occurs. To eliminate this drawback, there is a need for
troublesome operations of stopping the supply of clean air every
transport of the wafer cassette.
[0119] On the other hand, the cleaning method and apparatus
according to an embodiment of the present invention are
characterized by cleaning the substrates while supplying the clean
air from the part above the cleaning bath toward the level of the
cleaning solution downwards vertically. Thus, since the clean air
flows in parallel to the surface of each wafer at the time of
transport of the wafer cassette, there is little fear of occurrence
of particles. Accordingly, the wafer cassette may be transported
with the clean air kept flowing.
[0120] Using the wet-cleaning method of the present embodiment
permits the cleaning process to be made uniform enough to provide
cleaned products of high quality stably, and also may decrease the
frequency in replacement of the cleaning solution. In consequence,
a burden on the waste fluid treatment may be lightened thanks to
the decrease in amount of released waste fluid of the used cleaning
solution with the decreasing amount of chemicals consumed for
preparing the cleaning solution. In addition, the resource saving
is attained with the decreasing amount of chemicals consumed for
the waste fluid treatment, resulting in the substantial decrease in
amount of sludge and waste water released from the waste fluid
treatment.
[0121] According to the wet-cleaning method of an embodiment of the
present invention, since the cleaning process has an increased
effect of decreasing the evaporation loss water from the cleaning
solution, and besides, the evaporation loss of water is controlled
within the predetermined range, the Linear relation is provided
stably between the time required for cleaning and the concentration
of the active components in the cleaning solution, providing more
increased effects of an embodiment of the present invention.
[0122] According to the wet-cleaning method of an embodiment of the
present invention, since the solution prepared by dissolving at
least one of ammonium fluoride and hydrofluoric acid in water is
used as the cleaning solution for cleaning in such a way that the
silicon oxide layer is removed from the surface of the silicon
substrate by etching, the cleaning process is made uniform enough
to provide the linear relation between the cleaning time and the
etching rate, permitting a highly reproducible cleaning process,
together with highly accurate control of the etching rate.
[0123] The wet-cleaning apparatus according to an embodiment of the
present invention may carry the cleaning method according to an
embodiment of the present invention into operation accurately, or
more accurately and efficiently, or more accurately and stably.
[0124] A description will now be given of the method and apparatus
for cleaning the substrate according to embodiments of the present
invention in detail with reference to the accompanying
drawings.
[0125] The present invention is directed to the process of cleaning
(or etching) the substrate with an aqueous solution of ammonium
fluoride or ammonium fluoride hydrofluoric acid mixture as a
cleaning solution, and more specifically, to the process of
cleaning the substrate with a cleaning solution of a fixed
concentration at all times through the process of replenishing the
cleaning solution with a required component such as water,
ammonium, aqueous ammonium, an aqueous solution of ammonium
fluoride depending on the change in concentration of various
components in the cleaning solution for correction of the changed
concentration, while replacing the cleaning solution depending on
the cumulative amount of time the cleaning solution was used.
[0126] FIG. 11 is a graphical representation of the relation
between the lapse of time and the etching rate to a thermal oxide
as to the ammonium fluoride hydrofluoric acid mixture solution. The
composition ratio of NH.sub.4F (40 wt. %) to HF (50 wt. %) in the
cleaning solution is 400/1, the temperature of the cleaning
solution is limited to 25 C., and the thermal oxide layer is
SiO.sub.2.
[0127] It is ascertained from FIG. 11 that the etching rate to the
thermal oxide substantially increases with the lapse of time.
[0128] Although the etching rate changes every moment in the
process of cleaning the substrate with the above cleaning solution,
there is a considerably high correlation (a proportional relation)
between the etching rate and the elapsed time. This means that the
water content or the chemical components (NH.sub.4F, HF) in the
cleaning solution change (evaporate) with the lapse of time, more
specifically, the HF (a direct component causing etching of the
thermal oxide) concentration of the cleaning solution increases at
a fixed rate by reason of evaporation of the water or the ammonium
component from the cleaning solution at a fixed rate.
[0129] FIG. 12 is a graphical representation of the change in HF
concentration of the cleaning solution with the lapse of time, and
it is ascertained from FIG. 12 that the HF concentration increases
at a fixed rate with the lapse of time.
[0130] Accordingly, it is considered that the process of cleaning
(etching) the substrates with the aqueous solution of ammonium
fluoride or ammonium fluoride hydrofluoric acid mixture needs to
maintain the HF concentration of the cleaning solution stably and
uniformly.
[0131] In this connection, according to an embodiment of the
present invention, there is provided a cleaning system as shown in
FIG. 13 to maintain the HF concentration of the cleaning solution
at a value within a predetermined range.
[0132] In the cleaning system, the concentration of the cleaning
solution is measured regularly for the subsequent operations on the
basis of the result of measurement.
[0133] A description will now be given of the cleaning system
applied to the case of measuring the HF concentration as a target
concentration for measurement.
[0134] As shown in FIG. 13, in the cleaning system of the present
embodiment, the HF concentration is measured regularly (every
6.times.n time, for instance; n being an integer less than 8) with
a concentration measuring unit since the HF concentration gradually
changes with the lapse of time as described the above (Step
132).
[0135] According to the cleaning system, measurement on the
component concentration (HF, for instance) in the cleaning solution
may be executed by employing a method such as measuring absorbance
of light with a predetermined wavelength, infrared/ultraviolet
absorption spectrum, refractive index, specific gravity, light
transmittance and conductivity or the like or using a measuring
unit such as a Karl Fischer's titration meter and a liquid (ion)
chromatography or the like.
[0136] If the HF concentration lies within the predetermined range,
that is, the range of 0.05 to 0.1 wt. % as the result of
measurement on the concentration, the cleaning process is started
(Steps 133, 134).
[0137] Then, the whole cleaning solution in the cleaning bath is
replaced after having been used for a predetermined period of time
up to the solution replacement time (Step S135). In this cleaning
system, the solution replacement time is set every lapse of 48
hours (Step 131).
[0138] When the replacement of cleaning solution is performed, the
concentration of the replaced solution is measured a predetermined
period of time, that is, 10 minutes after the completion of the
replacement of cleaning solution.
[0139] The cleaning system continues cleaning until the solution
replacement time is reached, while counting the elapsed time since
the concentration was measured. Then, the HF concentration is
ascertained through re-measurement on the concentration after the
lapse of a predetermined period of time (every 6.times.n hours, for
instance; n being an integer less than 8) (Step 136).
[0140] Incidentally, in the above flow, it does not make any
difference on which ascertain earlier, the solution replacement
time or the elapsed time since the concentration was measured.
[0141] On the other hand, when the HF concentration is out of the
predetermined range, that is, the range of 0.05 to 0.1 wt. %, for
instance, as the result of measurement on the concentration, the
cleaning solution is replenished with a component for correcting
the HF concentration (Step 138).
[0142] If the measured HF concentration exceeds the predetermined
range described the above, for instance, the cleaning solution is
replenished with the ammonium component to lower the HF
concentration (Step 139).
[0143] In this step, supplying the ammonium component corresponding
to the molar number of a difference between the set HF
concentration and the measured HF concentration permits the HF
concentration to be restored to a value within the above
predetermined range.
[0144] Specifically, the cleaning solution is replenished with
ammonium gas or aqueous ammonium containing the ammonium component
in an amount given by the expression of [Measured HF concentration
(wt. %)-set HF concentration (wt. %)].times.(ammonium molecular
weight/HF molecular weight).times.total weight of cleaning
solution.
[0145] Replenishing with the ammonium component causes the
neutralization of HF to provide ammonium fluoride, and as a result,
the HF concentration in the cleaning solution is controlled.
Replenishing with aqueous ammonium may provide the effects on both
dilution and neutralization.
[0146] On the other hand, when the measured HF concentration is
lower than the predetermined range, the cleaning solution is
replenished with the HF component to raise the HF
concentration.
[0147] In this step, supplying the HF component corresponding to a
difference between the set HF concentration and the measured HF
concentration permits the HF concentration to be restored to a
value within the above predetermined range.
[0148] Specifically, the cleaning solution is replenished with the
HF component such as HF gas and hydrofluoric acid in an amount
given by the expression of [Measured HF concentration (wt. %)-set
HF concentration (wt. %)].times.total weight of cleaning
solution.
[0149] Incidentally, in the cleaning system, an error message is
issued to temporarily discontinue the operation, if the HF
concentration is not brought within the predetermined range in
spite of several times of the measurement on concentration.
Specifically, the cleaning system issues the error message to
temporarily discontinue the operation, if the HF concentration was
not brought within the predetermined range four times in
succession, for instance (Step 137).
[0150] Replenishing with the component for correction of the HF
concentration if being out of the predetermined range as the result
of regular measurement on the HF concentration makes the etching
process with the cleaning solution uniform and stable (that is,
maintains the etching rate to the oxide constant), eliminating the
need for frequent replacement of the cleaning solution like the
related art. Thus, the life of the cleaning solution may be
extended, permitting a contribution toward not only the decrease
(resource saving) in cleaning solution consumption with the
decreasing frequency in solution replacement but also the decrease
(resource saving) in waste water treatment agents for the waste
water treatment for the cleaning solution. A contribution toward
the decrease in amount of sludge and waste water released from the
waste water treatment (the decrease in released waste amount) with
the decreasing amount of chemicals consumed is also possible.
[0151] Thus, it may be said that the embodiment of the present
invention permits the substrate cleaning process to be made uniform
and stable, but also may contribute toward the preservation of the
global environment inclusive of the resource saving and the
decrease in released waste amount or the like.
[0152] As described the above, using the method and apparatus of
the present embodiment permits the process of cleaning the
substrate with the aqueous solution of ammonium fluoride to be made
uniform and stable and also may decrease the frequency in
replacement of the cleaning solution, permitting not only the
resource saving of chemicals (the cleaning solution and the waste
water treatment agent) but also the substantial decrease in
released sludge and waste water amounts with the decreasing amount
of chemicals consumed.
[0153] A description will now be given of embodiments of the
present invention with reference to the accompanying drawings.
[0154] FIG. 14 is a schematic illustration of a structure of a
wet-cleaning apparatus according to one embodiment of the present
invention, together with a substrate cleaning method carried into
operation using this wet-cleaning apparatus. The cleaning apparatus
has a SC-1 bath 151, a rinsing bath 152, a SC-2 bath 153, a rinsing
bath 154, a BHF bath 155, a rinsing bath 156, a rinsing bath 157
and a drying bath 158, which are arranged in this order side by
side.
[0155] A cleaning solution prepared by dissolving ammonium and
aqueous hydrogen peroxide (H.sub.2O.sub.2) in water is stored in
the SC-1 bath 151, while a cleaning solution prepared by dissolving
hydrochloride acid and aqueous hydrogen peroxide (H.sub.20.sub.2)
in water is stored in the SC-2 bath 153. Incidentally, the above
ammonium is dissolved in the form of ammonium hydrate (NH.sub.4OH).
A cleaning solution (normally having the HF concentration of about
0.1 wt. % and the NH.sub.4F concentration of about 40 wt. %)
prepared by dissolving ammonium fluoride (NH.sub.4F) and
hydrofluoric acid (HF) in water is stored in the BHF bath 155,
while pure water (ion-exchange water) is stored in each of the
rinsing baths 152, 154, 156 and 157. The drying bath 158, which may
be for drying speedily the surface of the substrate by blowing
heated clean air thereagainst for exhalation of IPA (isopropyl
alcohol) after spraying steam of IPA toward the surface of the
rinsed substrate, is used.
[0156] A description will now be given of a structure of an
automatic diluent supply unit for restoring the HF concentration to
a value set earlier than the rise of HF concentration by diluting
the cleaning solution in the BHF bath 155 with a predetermined
amount of cleaning solution intermittently supplied from the SC-1
bath 151 depending on the amount of cleaning solution in the BHF
bath 155 and the rise of HF concentration . That is, the SC-1 bath
151 and the BHF bath 155 are connected together through piping 22
provided with a pump 161. The discharge side of the pump 161 of the
piping 162 is provided with an electromagnetic valve and an
integrating flow meter, which are connected together. The BHF bath
155 is provided with a cleaning solution level gauge and an HF
concentration measuring unit for measuring the HF concentration of
the automatically sampled cleaning solution. There is also provided
an automatic control unit for automatically opening the
electromagnetic valve for a predetermined period of time. Further,
the level gauge and the HF concentration measuring unit are
connected to the automatic control unit.
[0157] A cassette transport mechanism for vertically or
horizontally transporting the wafer cassette is provided as
extending from a part above the SC-1 bath 151 to a part above the
drying bath 158. Further, the whole wet-cleaning apparatus is
installed in a chamber partitioned with a wall as desired in such a
way as to supply the clean air from the part above the SC-1 bath
151 while exhausting air from the part above the drying bath
158.
[0158] A description will now be given of one embodiment of a
silicon substrate cleaning method carried into operation using the
above substrate cleaning apparatus. A number of silicon substrates
(silicon wafers) are loaded in a wafer cassette in such a way that
the substrates are placed vertically in parallel to each other at
proper spaces. The cassette is supported with the transport
mechanism and is then lowered into the SC-1 bath 151 to immerse the
silicon substrates inclusive of the cassette in the cleaning
solution for cleaning. The silicon substrates in the cleaning
process are cleaned in such a way that the thermal oxide or native
oxide layer is removed from the surface of each silicon substrate
by etching. After the cleaning process, the cassette is pulled up
by the transport mechanism for transport to a portion right above
the rinsing bath 152. Then, the cassette is lowered into the
rinsing bath to immerse the silicon substrates inclusive of the
cassette in pure water for rinsing. Thereafter, the similar
procedure is required to clean and rinse the silicon substrates in
the SC-2 bath 153 and the rinsing baths 154 to 157, and finally,
the rinsed silicon substrates are dried in the drying bath 158.
[0159] In the cleaning apparatus, a predetermined amount of
cleaning solution (with no content of HF) in the SC-1 bath 151 is
intermittently supplied into the BHF bath 155 to dilute the HF
concentration of the cleaning solution in the BHF bath 155 with
water in SC-1 solution for the purpose of restraining the HF
concentration of the cleaning solution in the BHF bath 155 from
rising with the lapse of cleaning time so as to maintain the HF
concentration substantially constant to provide the etching rate
maintained substantially constant. The intermittent supply of
diluent is carried out according to the following procedure.
Incidentally, the pump 161 is kept operative for the duration of
the substrate cleaning process.
[0160] The level of the cleaning solution in the BHF bath 155 is
measured intermittently with the level gauge at intervals of a
predetermined period of time, and the measured value is transmitted
to the automatic control unit. In synchronization with the level
measurement, the HF concentration of the cleaning solution is
measured with the HF concentration measuring unit, and this
measured value is also transmitted to the automatic control unit.
In the automatic control unit, the amount of cleaning solution in
the BHF bath 155 is calculated on the basis of the measured level
value, and the required amount of diluent supplied is calculated on
the basis of the calculated amount of cleaning solution and the
measured HF concentration value. Then, the electromagnetic valve is
opened in response to a signal from the automatic control unit to
start the supply of diluent. When the fact that the required amount
of diluent supplied is reached is measured with the integrating
flowmeter, a signal representing the above fact is transmitted to
the automatic control unit. The automatic control unit then closes
the electromagnetic valve on the basis of this transmitting signal,
thereby terminating one cycle of intermittent supply of diluent.
Thereafter, the intermittent supply of diluent is repeated
according to the similar procedure.
[0161] In the cleaning method and apparatus according to an
embodiment of the present invention, diluents available for
diluting the HF concentration include aqueous solutions of ammonium
aqueous hydrogen peroxide mixture (SC-1: the mixed aqueous
solution), hydrochloric acid aqueous hydrogen peroxide mixture
(SC-2), sulfuric acid aqueous hydrogen peroxide mixture, phosphoric
acid, sulfuric acid nitric acid mixture, fuming nitric acid and
acetic acid, ion exchange water and hydrophilic organic solvents
[alcohol [isopropyl alcohol, ethyl alcohol, methyl alcohol,],
acetone, dimethyl sulfoxide (DMSO), N-methylpyrrolidene (NMP),
dioxane, and dimethylformamide (DMF)]. These diluents may be used
individually or as a mixture of appropriately selected
diluents.
[0162] Among the above diluents, the aqueous solution of ammonium
aqueous hydrogen peroxide mixture (SC-1), ion exchange water and
alcohol are suitable for the diluent. When SC-1 is supplied, the
above ammonium component has effects of converting the HF component
in the target cleaning solution for supply of SC-1 into NH.sub.4F
and also diluting the HF component with water and aqueous hydrogen
peroxide in the diluent. As described the above, SC-1 is supposed
to be a diluent having a high effect on dilution of the HF
concentration.
[0163] Further, the above diluent may be a virgin product or a
recycled product once used for the other object. From the viewpoint
of the resource saving, the latter is preferable. The supply of
diluent may be carried out concurrently with the operation of
replenishing with ammonium fluoride, hydrofluoric acid, ammonium,
water and a surfactant or the like as the components of the target
cleaning solution for dilution of the HF concentration.
[0164] A description will now be given of a structure of a
wet-cleaning apparatus according to another embodiment, together
with a substrate cleaning method carried into operation using this
wet-cleaning apparatus with reference to FIG. 15. This wet-cleaning
apparatus is similar in structure to that shown in FIG. 14, except
that a rinsing bath 172 and a BHF bath 175 are connected together
through piping 182 provided with a pump 181 to dilute the HF
concentration of the cleaning solution in the BHF bath 175 with
pure water (ion exchange water) in the rinsing bath 176.
[0165] A description will now be given of a structure of a
wet-cleaning apparatus according to a further embodiment, together
with a substrate cleaning method carried into operation using this
wet-cleaning apparatus with reference to FIG. 16. This wet-cleaning
apparatus is similar in structure to that shown in FIG. 14, except
that a drying bath 208 and a BHF bath 205 are connected together
through piping 212 provided with a pump 201. In this case, an IPA
supply pipe installed at the drying bath 208 is connected to the
piping 212. According to this cleaning apparatus, the rise of HF
concentration of the cleaning solution is controlled with liquid
IPA supplied into the BHF bath 205.
[0166] A description will now be given of a structure of a
wet-cleaning apparatus according to a still further embodiment,
together with a substrate cleaning method carried into operation
using this wet-cleaning apparatus with reference to FIG. 17. This
cleaning apparatus is similar in structure to that shown in FIG.
14, except that a SC-1 bath 221 and a BHF bath 225 are connected
together through piping 232a provided with a pump 231a, and a
rinsing bath 226 and the BHF bath 225 are connected together
through piping 232b provided with a pump 231b. According to this
cleaning apparatus, the HF concentration of the cleaning solution
in the BHF bath 225 is diluted with water in the SC-1 bath 221 and
pure water in the rinsing bath 226.
[0167] A description will now be given of a structure of a
wet-cleaning apparatus according to a yet further embodiment,
together with a substrate cleaning method carried into operation
using this wet-cleaning apparatus with reference to FIG. 18. This
cleaning apparatus is quite different from those shown in FIGS. 14
to 17 in structure and operation. That is, in use of two cleaning
apparatuses of different systems, a cleaning solution (with no
content of HF) of one cleaning apparatus 240 and liquid IPA are
supplied into a cleaning solution in the other cleaning apparatus
250 to control the rise of HF concentration so as to maintain the
HF concentration substantially constant.
[0168] Specifically, the cleaning apparatus 240 has a SC-1 bath
241, a rinsing bath 242, a SC-2 bath 243, a rinsing bath 244, a DHF
bath 245, a rinsing bath 246, a rinsing bath 247 and a drying bath
248, which are arranged in this order in a string. The cleaning
apparatus 250 has a NH.sub.4F bath 251, a BHF bath 252, a DHF bath
253, rinsing baths 254, 255 and a drying bath (a spin drier) 256,
which are arranged in this order in a string. The SC-1 bath 241 of
the cleaning apparatus 240 is connected to the NH.sub.4F bath
through piping 258a provided with a pump 257a, and the drying bath
248 of the cleaning apparatus 240 is connected to the DHF bath 253
through piping 258b provided with a pump 257b. The spin drier 256,
which may be for spinning the substrate at high speed to shake off
condensed IPA by centrifugal force after the IPA steam is sprayed
against the substrate surface on condition that the substrate
having been rinsed with pure water is set as being freely spun at
high speed about a vertical axis, is used.
[0169] Incidentally, the supply of pure water from the rinsing tank
(242, for instance) of the cleaning apparatus 240 into the
NH.sub.4F bath 251 and the DHF bath 253 will do, instead of the
supply of SC-1 solution and liquid IPA in FIG. 18. Also, the supply
of pure water from the rinsing bath 254 of the cleaning apparatus
250 is also applicable, in addition to the embodiment of FIG.
18.
[0170] FIG. 19 is a graphical representation of the change in
silicon oxide etching rate with the lapse of time and the change in
HF concentration of the cleaning solution with the lapse of time
according to the results of experiments in use of the cleaning
method (example) of an embodiment of the present invention, in
comparison with those according to the results of experiments in
use of the cleaning method in the related art. According to the
example of the present embodiment, a predetermined amount of
predetermined diluent was intermittently supplied by use of the
cleaning apparatus of FIG. 14 to control the rise of HF
concentration of the cleaning solution in the BHF bath 155. On the
other hand, according to the comparative example, cleaning was
performed without the supply of diluent at all. As is apparent from
FIG. 19, the etching rate and the HF concentration according to the
embodiment are maintained substantially constant irrespective of a
difference in the elapsed time. On the other hand, the etching rate
and the HF concentration according to the comparative example are
gradually increasing.
[0171] Incidentally, measurement on the concentration of active
components and water in the cleaning solution may be by methods for
measuring (1) absorbance of light with a predetermined wavelength,
(2) infrared/ultraviolet absorption spectrum, and (3) refractive
index, specific gravity, light transmittance and conductivity or
the like, or using a measuring unit such as (4) a Karl Fischer's
titration meter and (5) a liquid (ion) chromatography or the
like.
[0172] Using the cleaning method and apparatus as described the
above may substantially lower the change or non-uniformity in
etching rate, that is, cleaning effects with the lapse of cleaning
time as to the cleaning solution prepared by dissolving at least
hydrofluoric acid as an active component in water. Thus, the
cleaning process with the cleaning solution may be made uniform and
stable enough to provide higher yield of the semiconductor
substrate and the liquid crystal display substrate, permitting the
substantial decrease in amount of chemicals for the waste fluid
treatment (the waste water treatment) for the cleaning solution
with the decreasing frequency in replacement of the cleaning
solution (the extended life of the cleaning solution). The above
cleaning method and apparatus also have an effect of decreasing the
amount of wastes released (that is, sludge and waste water
(sewage)) with the decreasing amount of chemicals consumed.
[0173] As has been described in the foregoing, according to the
wet-cleaning method and apparatus of the embodiments of the present
invention, the cleaning process is made uniform enough to provide
cleaned products of high quality stably, as well as higher yield of
the semiconductor substrate and the liquid crystal display
substrate, permitting the decrease in frequency in replacement of
the cleaning solution. As a result, a burden on the waste fluid
treatment may be lightened thanks to the decrease in amount of
released wastes of the used cleaning solution with the decreasing
amount of chemicals consumed for preparing the cleaning solution.
In addition, the resource saving is attained with the decreasing
amount of chemicals consumed for the waste fluid treatment,
resulting in the decrease in amount of sludge and waste water
released from the waste fluid treatment.
[0174] A description will now be given of a substrate cleaning
method and a substrate cleaning apparatus according to a further
embodiment of the present invention with reference to the
accompanying drawings.
[0175] The present invention is directed to the process of cleaning
(or etching)the substrate with an aqueous solution of ammonium
fluoride or ammonium fluoride hydrofluoric acid mixture as a
cleaning solution, and more specifically, to the process of
replenishing the cleaning solution with an ammonium component
depending on the cumulative amount of time the cleaning solution
was used and the concentration of various components in the
cleaning solution.
[0176] FIG. 20 is a graphical representation of the relation
between the elapsed time and the etching rate to a thermal oxide as
to the ammonium fluoride hydrofluoric acid mixture solution. The
composition ratio of NH.sub.4F (40 wt %) to HF (50 wt. %) is 400/1,
the temperature of the cleaning solution is limited to 25 C, and
the thermal oxide layer is SiO.sub.2.
[0177] It is ascertained from FIG. 20 that the etching rate to the
thermal oxide substantially increases with the lapse of time.
[0178] As described the above, while the etching rate changes every
moment in the process of cleaning the substrate with the above
cleaning solution, there is a considerably high correlation (a
proportional relation) between the etching rate and the elapsed
time. This means that the water content or chemical components
(NH4F, HF) in the cleaning solution change (evaporate) with the
lapse of time, more specifically, the concentration of the HF
component (the direct component causing etching of the thermal
oxide) increases at a fixed rate by reason of the evaporation of
water or ammonium component from the cleaning solution at a fixed
rate.
[0179] FIG. 21 is a graphical representation of the change in HF
concentration of the cleaning solution with the lapse of time, and
it is ascertained that the HF concentration increases at a fixed
rate with the lapse of time.
[0180] In this embodiment, how the ratio of NH.sub.4F to HF
[NH.sub.4F(wt. %)/HF(wt. %)] changes with the lapse of time was
examined using the aqueous solution of ammonium fluoride
hydrofluoric acid mixture. The results are shown in FIG. 22.
[0181] It is ascertained from FIG. 22 that the component change
with the lapse of time varies depending on the ratio of ammonium
fluoride to hydrogen fluoride.
[0182] Specifically, when the composition ratio of ammonium
fluoride (wt. %) to hydrogen fluoride (wt. %) is not more than 50
(.ltoreq.50), the change in composition ratio with the lapse of
time hardly occurs. On the other hand, when the composition ratio
of ammonium fluoride (wt. %) to hydrogen fluoride (wt. %) is more
than 50 (>50), the change in composition ratio with the lapse of
time occurs (lowers). This is because the cleaning solution
containing a large percentage of ammonium fluoride is easily
affected by the evaporation of ammonium component as shown by the
expression of NH.sub.4F NH.sub.3 t +HF. (It is a matter of course
that evaporation of water occurs in this case as well.)
[0183] As described the above, it may be said that the means of
replenishing the cleaning solution with the ammonium component at
any time has an effect of maintaining the cleaning effects (the
etching rate) constant when the composition ratio of ammonium
fluoride (wt. %) to hydrogen fluoride (wt. %) is larger than 50
(>50) in the process of cleaning (etching) the substrate with
the ammonium fluoride hydrofluoric acid mixture solution.
[0184] That is, replenishing with the ammonium component with the
lapse of time may control the rise of HF concentration enough to
maintain the etching rate constant.
[0185] As a result, using this cleaning method may provide higher
yield of the semiconductor substrate and the liquid crystal
substrate and also may decrease the frequency in solution
replacement, permitting the decrease in released sludge and waste
water amounts with the decreasing amount of chemicals consumed,
such as the cleaning solution and the waste water treatment
agent.
[0186] Incidentally, the ammonium component to be replenished may
be gaseous or liquid. Also useful is aqueous ammonium diluted with
water (pure water) at a desired concentration or solutions of
hydrophilic solvents (acetone and various kinds of alcohol or the
like) solvable with ammonium component, or a mixture with ammonium
fluoride.
[0187] Incidentally, evaporation of water content, in addition to
the ammonium component, from the cleaning solution occurs. For that
reason, replenishing with a proper amount of water (pure water), in
addition to the ammonium component, will be also enough to maintain
the exactly same composition as that at the initial time of
cleaning. Incidentally, since the etching rate to each oxide is
less affected by water content (whereas being largely affected by
the HF concentration), replenishing with only the ammonium
component is no matter. However, in this case, while the HF
concentration of the cleaning solution is maintained constant by
means of replenishing with the ammonium component, the NH.sub.4F
concentration is gradually increasing with the lapse of time due to
the evaporation of water content.
[0188] A description will now be given of a substrate cleaning
apparatus according to a further embodiment of the present
invention.
[0189] FIG. 23 is a view illustrating a substrate cleaning
apparatus according to the further embodiment of the present
invention.
[0190] This substrate cleaning apparatus comprises a substrate
cleaning bath 271 stored with a cleaning solution such as an
ammonium fluoride hydrofluoric acid mixture solution f or cleaning
the substrate and a circulation pump 272 for circulating the
overflowing cleaning solution to the above substrate cleaning bath
271. The substrate serving as a target for cleaning is loaded in a
substrate carrier, for instance, for immersing the substrate
inclusive of the substrate carrier in the substrate cleaning bath
271 for cleaning (etching).
[0191] The features of the substrate cleaning apparatus are that
the substrate cleaning bath 271 is provided with an aqueous
ammonium reservoir tank 277 for supply of an ammonium component,
and the amount of ammonium component replenished is controlled by a
control unit 274.
[0192] Incidentally, ammonium component supply means available
other than the above includes a continuous flow pump and means of
gravity dropping, forcible feed with air and gas such as nitrogen
and so on.
[0193] As described the above, the etching rate to the thermal
oxide substantially increases with the increasing HF component in
the cleaning solution with the lapse of time at a fixed rate.
[0194] In this connection, according to the substrate cleaning
apparatus, the aqueous ammonium is supplied from the aqueous
ammonium reservoir tank 277 to the substrate cleaning bath 271 with
the control unit 274 with the lapse of time to maintain the HF
component rate constant enough to provide the etching rate
maintained substantially constant.
[0195] The optimum conditions on the amount of ammonium component
replenished and the replenishing timing may be set up by acquiring
data as shown in FIGS. 20, 21 and 22 for calculation of the HF
component increasing with the lapse of time.
[0196] Incidentally, it is preferable that the cleaning solution is
replenished with the ammonium component in an amount corresponding
to the evaporation loss till a desired period of time enough to
maintain the cleaning solution at the (initial) concentration
provided immediately after the preparation of the cleaning
solution. The amount of ammonium component added may be calculated
on the basis of the concentration of the cleaning solution
resulting from continuous or intermittent monitoring, for
instance.
[0197] FIG. 24 is a graphic representation of the controlled state
of the etching rate and the HF concentration depending on
replenishment with the ammonium component. The etching rate and the
HF concentration are maintained within the fixed range by means of
intermittently replenishing with the ammonium component.
[0198] Incidentally, either continuous replenishment or
intermittent replenishment will be enough for the conditions of
replenishment with the ammonium component. The former is more
suitable to maintain the concentration constant at all times.
[0199] While the above substrate cleaning apparatus is configured
to control the timing to replenish with the ammonium component on
the basis of the measurement data, an apparatus, which is provided
with a concentration measuring unit 285 as shown in FIG. 25 for
measurement on the concentration of circulating cleaning solution
to control replenishment with the ammonium component in real time
on the basis of measured concentration information, is also
applicable. The concentration measuring unit which is installed as
a standing unit in the cleaning apparatus to feed back the result
of measurement during the cleaning process or has the effect of
reflecting the results measurement in spots to add the ammonium
component under predetermined conditions without the need for being
installed as the standing unit will be also used.
[0200] Specifically, control of the replenishment with the ammonium
component is by the process of measuring the concentration of each
component (ammonium fluoride, hydrogen fluoride and water content
or the like) in the cleaning solution with the concentration
measuring unit 285, then transmitting the result of measurement to
a control unit 284 composed of a computer or a central monitoring
board or the like for calculation of the required mount of ammonium
component replenished subsequent to the judgment on the need or not
of replenishment, and thereafter sending an instruction on
replenishment to an aqueous ammonium feed line (the aqueous
ammonium reservoir tank 287). Then, whether or not replenishment
with the ammonium component was made in accordance with the
instruction is ascertained by measurement with the concentration
measuring unit 285.
[0201] Incidentally, measurement on the concentration of components
in the cleaning solution may be by methods for measuring absorbance
of light with a predetermined wavelength, infrared/ultrasonic
absorption spectrum, refractive index, specific gravity, light
transmittance and conductivity or the like or using a measuring
unit such as a Karl Fischer's titration meter and a liquid (ion)
chromatography or the like.
[0202] As described the above, replenishing the cleaning solution
with the ammonium component on the basis of the concentration of
various components or the etching rate at each point of the
cleaning process permits the etching process with the cleaning
solution to be made uniform and stable (that is, maintains the
etching rate constant), eliminating the need for frequent
replacement of the cleaning solution like the related art. Thus,
the life of the cleaning solution may be extended, permitting a
contribution toward not only the decrease (resource saving) in
cleaning solution consumption with the decreasing frequency in
solution replacement but also the decrease (resource saving) in
waste water treatment agent for the waste water treatment for the
cleaning solution. A contribution toward the decrease in amount of
sludge and waste water released from the waste water treatment (the
decrease in released waste amount) with the decreasing amount of
chemicals consumed is also possible.
[0203] As described the above, the present embodiment not only
permits the substrate cleaning process to be made uniform and
stable but also may contribute toward the preservation of the
global environment inclusive of the resource saving and the
decrease in released waste amount.
[0204] As described the above, using the method and apparatus of
the present embodiment permits the process of cleaning the
substrate with the aqueous solution of ammonium fluoride to be made
uniform and stable and also may decrease the frequency in
replacement of the cleaning solution, permitting the substantial
decrease in released sludge and wastewater amounts with the
decreasing amount of chemicals consumed (the resource saving of the
cleaning solution and waste treatment agents).
[0205] A description will now be given of a method and apparatus
for cleaning a substrate according to the still further embodiment
of the present invention with reference to the accompanying
drawings.
[0206] The present invention relates the process of cleaning (or
etching) the substrate with an aqueous solution of ammonium
fluoride hydrofluoric acid mixture as a cleaning solution, and more
specifically, to the process of replenishing the cleaning solution
with water depending on the cumulative time the cleaning solution
was used and the concentration of various components in the
cleaning solution.
[0207] FIG. 26 is a graphic representation of the relation between
the elapsed time and the etching rate to the thermal oxide as to
the ammonium fluoride hydrofluoric acid mixture solution. The
composition ratio of NH.sub.4F (40 wt. %) to HF (50 wt. %) in the
cleaning solution is 400/1, the temperature of the cleaning
solution is limited to 25 C., and the thermal oxide layer is
SiO.sub.2.
[0208] It is ascertained from FIG. 26 that the etching rate to the
thermal oxide substantially increases with the lapse of time.
[0209] As described the above, while the etching rate changes every
moment in the process of cleaning the substrate with the above
cleaning solution, there is a considerably high correlation between
the etching rate and the elapsed time. This means that the water
content and components of chemicals (NH.sub.4F, HF) in the cleaning
solution change (evaporate) with the lapse of time, specifically,
the concentration of the HF component (the direct component causing
etching of the thermal oxide) in the cleaning solution increases at
a fixed rate by reason of the evaporation of water and ammonium
component from the cleaning solution.
[0210] FIG. 27 is a graphic representation showing the change in HF
concentration of the cleaning solution with the lapse of time, and
it is ascertained from FIG. 27 that the HF concentration increases
at a fixed rate with the lapse of time.
[0211] In this embodiment, how the ratio of NH.sub.4F to HF
[NH.sub.4F (wt. %) /HF (wt. %) changes with the lapse of time is
examined using the aqueous solution of ammonium fluoride
hydrofluoric acid mixture. The results are shown in FIG. 28.
[0212] It is ascertained from FIG. 28 that the component change
with the lapse of time varies depending on the ratio of ammonium
fluoride to hydrogen fluoride.
[0213] Specifically, when the ratio of ammonium fluoride (wt.) to
hydrogen fluoride (wt. %) is not more than 50 (.ltoreq.50), the
change in composition ratio with the lapse of time hardly occurs.
On the other hand, when the ratio of ammonium fluoride (wt. %) to
hydrogen fluoride (wt. %) is more than 50 (>50), the change in
composition ratio with the lapse of time occurs (lowers). This is
because the cleaning solution containing a high percentage of
ammonium fluoride is easily affected by the evaporation of ammonium
component as given by the expression of
NH.sub.4F.fwdarw.NH.sub.3.Arrow-up bold.+HF. (It is a matter of
course that evaporation of water occurs in this case as well.)
[0214] As described the above, it may be said that the means of
replenishing the cleaning solution with water at any time has an
effect of maintaining the cleaning effects (the etching rate)
constant when the ratio of ammonium fluoride (wt. %) to hydrogen
fluoride (wt. %) is not more than 50 (.ltoreq.50) in the process of
cleaning (etching) the substrate with the ammonium fluoride
hydrofluoric acid mixture solution.
[0215] That is, replenishing with water with the lapse of time may
control the rise of HF concentration enough to maintain the etching
rate constant.
[0216] As a result, using the above cleaning method may provide
higher yield of the semiconductor substrate and the liquid crystal
substrate or the like and also may decrease the frequency in
solution replacement, permitting the decrease in released sludge
and waste water amounts with the decreasing amount of chemicals
consumed, such as the cleaning solution and the waste water
treatment agent.
[0217] Incidentally, water to be replenished may be gaseous (steam)
or liquid. Also useful is solid (ice) or a mixture with a
hydrophilic solvent (acetone and various kinds of alcohol or the
like) dissolvable with water (pure water) at a desired rate.
[0218] FIG. 29 is a view illustrating a substrate cleaning
apparatus according to a still further embodiment of the present
invention.
[0219] This substrate cleaning apparatus comprises a substrate
cleaning bath 291 stored with a cleaning solution such as an
ammonium fluoride hydrofluoric acid mixture solution for cleaning
the substrate and a circulation pump 292 for circulating the
overflowing cleaning solution to the substrate cleaning bath 291. A
substrate as a target for cleaning is loaded in a substrate
carrier, for instance, to immerse the substrate inclusive of the
carrier in the substrate cleaning bath 291 for cleaning
(etching).
[0220] The features of the above apparatus are that the substrate
cleaning bath 291 is provided with a continuous flow pump 293 for
supply of water, and the amount of water replenished is controlled
by a control unit 294.
[0221] Incidentally, water supply means available other than the
above pump includes a means of gravity dropping forcible feed of
air and gas such as nitrogen and so on.
[0222] As described the above, the etching rate to the thermal
oxide substantially increases with the increasing concentration of
the HF component in the cleaning solution with the lapse of time at
a fixed rate.
[0223] In this connection, according to the above apparatus, water
is supplied into the substrate cleaning bath 1 through the
continuous flow pump 293 with the lapse of time by use of the
control unit 294 to maintain the HF concentration constant enough
to provide the etching rate maintained substantially constant.
[0224] FIG. 30 is a graphic representation of the change in HF
concentration depending on replenishment with water, and it is
ascertained that the HF concentration increasing with the lapse of
time is restored to its initial concentration value by means of
replenishing with water. Since control of the HF concentration by
means of replenishing with water is related to control of the
etching rate, the etching rate is also lowered by means of
replenishing with water as shown in FIG. 31.
[0225] The optimum conditions on the amount of water replenished
and the replenishing timing may be set up by acquiring data as
shown in FIGS. 26 and 27 for calculation of the evaporation loss of
water with the lapse of time.
[0226] Incidentally, it is preferable that the cleaning solution is
replenished with water in an amount corresponding to the
evaporation loss till a desired period of time enough to maintain
the cleaning solution at the (initial) concentration provided
immediately after the preparation of the cleaning solution. The
amount of water added may be calculated on the basis of the
concentration of the cleaning solution resulting from continuous or
intermittent monitoring, for instance.
[0227] FIG. 32 is a graphic representation of the controlled state
of the etching rate and the HF concentration depending on the
replenishment with water. The etching rate and the HF concentration
are maintained within the fixed range by means of intermittently
replenishing with water.
[0228] Incidentally, either continuous replenishment or
intermittent replenishment will be enough for the conditions on
replenishment with water. The former is more suitable to maintain
the concentration constant at all times.
[0229] While the above substrate cleaning apparatus is configured
to control the timing to replenish with water on the basis of the
measurement data, an apparatus, which is provided with a
concentration measuring unit 305 as shown in FIG. 33 for
measurement on the concentration of circulating cleaning solution
to control replenishment with water in real time on the basis of
the measured concentration information, is also applicable. The
concentration measuring unit which is installed as a standing unit
in the cleaning apparatus to feedback the result of measurement
during the cleaning process, or has the effect of reflecting the
results measured in spots to add water under predetermined
conditions without the need for being installed as the standing
unit, will be also used.
[0230] Specifically, control of the replenishment with water is by
the process of measuring the concentration of each component
(ammonium fluoride, hydrogen fluoride and water content or the
like) in the cleaning solution with the concentration measuring
unit 305, then transmitting the result of measurement to a control
unit 304 composed of a computer and a central monitoring board or
the like for the calculation of the required amount of water
replenished subsequent to the judgement on the need or not of
replenishment, and thereafter, sending an instruction on
replenishment to a water supply line (the continuous flow pump
303). Then, whether or not replenishment with water was made in
accordance with the instruction is ascertained by measurement with
the concentration measuring unit 305.
[0231] In this embodiment, measurement on the concentration of the
components in the cleaning solution may be by methods for measuring
absorbance of light with a predetermined wavelength,
infrared/ultrasonic spectrum, refractive index, specific gravity,
light transmittance and conductivity or the like or using a
measuring unit such as the Karl Fischer's titration meter and the
liquid (ion) chromatography or the like.
[0232] As described the above, replenishing the cleaning solution
with water on the basis of the etching rate and the concentration
of various components at each point of the cleaning process permits
the etching process with the cleaning solution to be made uniform
and stable (that is, maintains the etching rate to the oxide
constant), eliminating the need for frequent replacement of the
cleaning solution like the related art. Accordingly, the life of
the cleaning solution may be extended, permitting a contribution
toward not only the decrease (resource saving) in cleaning solution
consumption with the decreasing frequency in solution replacement
but also the decrease (resource saving) in waste water treatment
agent for the waste water treatment for the cleaning solution. A
contribution toward the decrease in amount of sludge and waste
water released from the waste water treatment (the decrease in
released waste amount) with the decreasing amount of chemicals
consumed is also possible.
[0233] As described the above, the present embodiment not only
permits the substrate cleaning process to be made uniform and
stable but also may contribute toward the preservation of the
global environment inclusive of the resource saving and the
decrease in released waste amount.
[0234] As has been described in the foregoing, using the method and
apparatus of the present embodiment permits the process of cleaning
the substrate with the ammonium fluoride hydrofluoric acid mixture
solution to be made uniform and stable and also may decrease the
frequency in replacement of the cleaning solution, permitting the
substantial decrease in released sludge and waste water amounts
with the decreasing amount of chemicals consumed (the resource
saving of the cleaning solution and the waste water treatment
agents)
[0235] Although the invention has been described in its preferred
form with a certain degree of particularity, obviously many
changes, variations and combinations are possible therein. It is
therefore to be understood that any modifications will be practiced
otherwise than as specifically described herein without departing
from the scope of the present invention.
* * * * *